Inactivation of a diol-epoxide and a K-region epoxide with high efficiency by glutathione transferase X.

Four glutathione transferases (EC 2.5.1.18), glutathione transferases A, B, and C and a hitherto unknown form, termed X, were purified to apparent homogeneity from rat liver cytosol. They were investigated for their abilities to inactivate two mutagenic epoxides derived from the polycyclic aromatic hydrocarbon benz(a)anthracene, the K-region epoxide benz(a)anthracene 5,6-oxide and the diol-epoxide r-8,t-9-dihydroxy-t-10,11-oxy-8,9,10, 11-tetrahydrobenz(a)anthracene. Mutagenic activity was determined using Salmonella typhimurium his- strain TA100. Glutathione alone had little if any influence on the mutagenicity of the diol-epoxide but significantly decreased the mutagenic effect of the K-region epoxide. This inactivation was enhanced by the addition of glutathione transferases. Both epoxides were inactivated by glutathione in the presence of each of the four enzymes, but with varying efficiencies. Inactivation of the K-region epoxide (in terms of its mutagenicity in the presence of glutathione) required extremely little enzyme, about 1000 times less than for the diol-epoxide. On a molar basis, glutathione transferase X (followed by C greater than A greater than or equal to B) was clearly the most efficient enzyme in inactivating both substrates and also more efficient than were three other purified enzymes (microsomal epoxide hydrolase, cytosolic epoxide hydrolase, and dihydrodiol dehydrogenase) previously investigated in this test system. Taking into account the amounts of enzyme present in rat liver, the glutathione transferases C and X were most effective in inactivating the epoxides examined. Thus, the newly discovered glutathione transferase X appears to be of substantial significance in the inactivation of two structural prototypes of epoxides derived from polycyclic aromatic hydrocarbons, a K-region epoxide and a non-bay-region vicinal diol-epoxide.